1. Field of the Invention
This invention relates to an image forming apparatus for forming an image on a recording medium.
2. Description of the Prior Art
In conventional image forming apparatuses based on an electrophotography system, an electrophotographic sensitive medium provided as an image carrier is charged by a charging device, and is irradiated with light in accordance with image information to form a latent image. This latent image is developed by a development device, and the developed image is transferred onto a sheet member or the like to form a recorded image.
Image forming apparatuses for forming color images are in demand. A type of image forming apparatus has therefore been proposed which has a plurality of image carriers each independently used with the above-described image forming process to form a full-color image. In such an apparatus, a yellow image, a magenta image, a cyan image and, preferably, a black image are formed on the image carriers and these color images are transferred at respective image carrier transfer positions to a sheet member so as to be superposed on each other.
This type of image forming apparatus is advantageous in terms of speeding recording since it has image forming sections each independently operated with a respective color. It is also adaptable for use with a particular sheet member such as a thick sheet or a transparent sheet, but entails a problem relating to suitably registering color images formed in different image forming sections. This registration is important because a misalignment of the positions of images in one or more of the four colors transferred onto the sheet member appears finally as a color misalignment or a change in color tone.
An apparatus having a plurality of photosensitive drums driven with one drive source has been disclosed to solve this problem (see U.S. Pat. No. 4,803,515). In this apparatus, the distances between photosensitive drums is selected to equalize the time interval at which a sheet member passes the photosensitive drums to an integer multiple of the period of a driving non-uniformity cycle of a drive source such as a driving motor. Consequently, the phases of deviations from the correct positions at the image forming sections due to driving non-uniformity are equalized, thereby preventing color misalignment. This arrangement is very effective in preventing color misalignment.
The present invention is an improvement on this type of apparatus and features pulleys to stabilize the drive meshing to improve the effect of limiting color misalignment.